Mutsumi Yoshida and Joerg Lahann* Departments of Chemical Engineering, Materials Science and Engineering, and Macromolecular Science and Engineering, 2300 Hayward Street, University of Michigan, Ann Arbor, Michigan 48109
On-demand switchable materials, such as surfaces that can alter their optical appearance, are ubiquitous in nature. For instance, cuttlefish, a subclass of cephalopods, are experts of camouflage, as they change their body patterns and colors depending on their surrounding environment via a visually driven, neurally controlled chromatophore apparatus (Figure 1).1 Chromatophores contain pigment granules that can be differentially dispersed throughout the cell; when the granules are concentrated in the center of the cell, the cell appears light in color, whereas when the granules are scattered throughout the cell, it appears dark.2 Granules are transported rapidly along the cytoskeletal network by motor proteins, and it is this speed by which these granules are transported that enables a quick change in body color. There are many other cases where nature uses time-control very effectively in switching critical properties. Although most synthetic materials still lack such switchability, new synthetic tools and enhanced fundamental understanding have brought such “smart” materials within reach of the materials research community. By definition, smart ma-
Although most synthetic materials still lack switchability, new synthetic tools and enhanced fundamental understanding have brought such ‘smart’ materials within reach of the materials research community. www.acsnano.org
terials are materials that can respond to external stimuli or their environment.3,4 Recent examples of smart materials include self-healing polymers that release healing agents upon structural damage,5,6 electronic paper displays, whose ink appears or disappears depending on electrical charge,7 and meta-materials that serve as an “invisibility cloak” to make objects invisible at a particular wavelength.8,9 Analogous to the camouflage behavior of cuttlefish, gels that change color in response to external stimuli such as temperature, pressure, humidity, and salt concentration have been fabricated in the laboratory.10 By altering the thickness of a thin film composed of alternating layers of two self-assembling polymers, polystyrene and poly(2-vinyl-pyridine), the refractive index of the layer is changed, thereby altering the visible color of the gel. Applications for these gels include colorimetric sensors that detect changes in external conditions, as well as display devices.10 Given the wide range of uses in nature, smart materials may have a wide range of potential applications in medicine, and many of them, including thermoresponsive hydrogels,11 switchable surfaces,12 and photoresponsive materials,13 have already been explored in this capacity. Another prominent motivation of the quest for synthetic materials with dynamically controlled properties is their potential use as dynamic blueprints in time-resolved self assembly. Despite this encouraging progress, the implementation of switchable properties will continue to be one of the central obstacles in nanomaterials science in the decades to come. Ultimately, imparting dynamic properties to nanomaterials will result in systems where the materials themselves will be the actual active device. In this issue of ACS Nano, Balazs and coworkers describe numerical simulations to predict the switching properties of a smart membrane material.14 The membrane comprises lipid bilayers in conjunction
ABSTRACT “Smart”
materialsOmaterials that respond to a stimulus or their environment to produce a dynamic and reversible change in critical propertiesOhave enabled progress in many areas, including display technologies, drug delivery, and self-healing materials for coating applications, among others. Many of the current examples
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Smart Nanomaterials
of smart materials are biomimetic, since nature employs and depends on dynamic and rapid switching for critical functions such as vision, camouflage, and ion channel regulation. Despite progress in designing smart materials and surfaces, much work is still needed in this area to increase their implementation in useful applications. In this Perspective, the challenges and outlook in this field are highlighted, including the work of Balazs and co-workers found in this issue of ACS Nano.
See the accompanying Article by Alexeev et al. on p 1117. *Address correspondence to
[email protected] Published online June 24, 2008. 10.1021/nn800332g CCC: $40.75 © 2008 American Chemical Society
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Figure 1. Camouflage of a cuttlefish that mimics its environment, in this case a checkerboard.1 Republished with permission from ref 1. Copyright 2006 Elsevier.
with amphiphilic Janus particles. In contrast to conventional lipid bilayers, lipid membranes embedded with Janus particles bearing hydrophilic and hydrophobic moieties form stable pores upon perforation by mechanical force. When opening the pore, Janus particles expose their hydrophilic portion to the solvent. As the mechanical force is reduced, particles that lined the pores migrate toward each other until they form stable clusters. Thus, the pore never completely closes and remains as a stable hole due to the self-assembled cluster of particles (Figure 2). If re-exposed to the initial stimulus, the pore can reopen, which is in clear contrast to lipid bilayers without anisotropic particles. As a consequence, the composite membrane can open and close on demand. In this way, a smart property has been imparted to the membrane, resulting in stimuliresponsive pores that can be
opened and closed controllably and reversibly. It can be expected that membranes such as the example that Balazs and co-workers present can selectively control transport of materials and may find potential use in self-healing materials, nanofluidics, and drug delivery. Fundamentally, the system studied by Balazs and co-workers is intriguing in that it combines an oriented interface, the lipid bilayer, with anisotropic particles. Thus far, activities toward smart materials have exploited two lines of design (Figure 3). The first approach is the design of surface coatings or bulk materials that undergo reversible molecular transitions depending on the presence or absence of a stimulus. Stimuli include changes in pH, salt content, temperature, the presence of a specific analyte, electrical or magnetic fields, or light. A second approach takes advantage of anisotropic particles that can change surface properties if oriented at an interface in a controlled manner.
Figure 2. Smart membrane system consisting of anisotropic particles embedded in a lipid bilayer. Adapted from ref 14. Copyright 2008 American Chemical Society.
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Conformation-Based Switching. A number of smart surfaces have been developed that can switch surface properties upon exposure to internal
or external stimuli. Well-known examples include stimuli-responsive polymers, such as poly(N-isopropyl acrylamide) (pNIPAAm). This thermoresponsive polymer has been widely explored for a variety of biomedical applications including drug delivery (recently reviewed in refs 15–19), modulation of cell adhesion and protein adsorption,20–22 and cell sheet engineering.23 Another class of biologically inspired thermoresponsive molecules is based on elastin-like polypeptides (ELPs). These polypeptides contain the valine-proline-glycine-X-glycine motif, where X may be any amino acid other than proline.24 The molecules are water-soluble at lower temperatures but reversibly aggregate at elevated temperatures. The switching can be induced by changes in temperature, pH, or ionic strength.25,26 Elastin-like polypeptides have been micropatterned to capture and to release other proteins fused to ELPs via hydrophobic interactions upon transition.27 In addition to temperature, light can also serve as a stimulus, providing an alternative to actuate the smart properties in applications where temperature changes may not be a viable option. Several chemically triggered systems that change wettability upon light irradiation include azobenzene,28 pyrimidine,29 O-carboxymethylated calix[4]resorcinarene,30 or spiropyran31 groups. More recently, shape-memory polymers that respond to UV light have been reported.13,32,33 Such polymers are composed of an elastic polymer network and a molecular switch, which determines their permanent shape and forms reversible crosslinks upon photostimulus, respectively.34 This polymer can change from its flat ribbonlike structure to a tight coil, then back to a ribbon, in response to light.13 Alternatively, electrochemical transformations of self-assembled monolayers (SAMs) have been explored to produce smart substrates. www.acsnano.org
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centro-symmetric particles. Successful methods include the use of a spinning disk,52,53 self-assembly,54,55 fusion of pre-existing particles,56 surface modification with partial masking,57–60 selective deposition,61,62 surface modification through partial contact with reactive media,63–65 microcontact printing,47,66 and template-assisted self-assembly.67,68 Other examples include heterodimer nanopartiFigure 3. Two potential approaches toward the design of smart materials. The orientation of anisotropic particles (left) as well as the conformation of molecular switches (right) are altered cles,69 solidified paraffin relative to a surface by application of a stimulus. The switching results in different surface droplets,70 and magproperties. netic microbeads.71 For For example, alteration in surface ing cleavage of the bulky end certain materials, dumbbell-, wettability has been shown upon group, a mercaptohexadecanoic snowman-, or acorn-like nanopartiapplication of electrical potentials acid SAM of low density resulted, al- cles have been developed. Known in combination with SAMs of allowing the hydrophobic chains to methods include controlled crystalkanethiolates on gold.35 In this sys- alter their conformation reversibly lization from precursor core⫺shell tem, a decrease in contact angle of in response to changes in the elecparticles72,73 and selected surface ⬎80° to 0° was observed. In another trical potential. This change in connucleation.74–76 In related work, approach, conformational switchformational transition led to Rainer et al. employed polystyreneing of surface-confined molecules changes in surface wettability, b-polybutadiene-b-poly(methyl was used to control interfacial prop- thereby allowing dynamic control methacrylate) and similar erties dynamically.36 Similarly, conof interfacial properties. Other elec- polystyrene-b-polybutadiene-btrol of molecule concentration spatrochemically driven switchable sur- poly(methacrylic acid) triblock cotially and temporally by taking face designs to control surface wet- polymers for the synthesis of Janus advantage of surfactant desorption tability include gold electrodes micelles.77,78 These triblock copoly37 has been reported. By using elec- coated with monolayers of mers have well-defined bulk mortrodes, surfactants bearing redoxbipyridinium-containing thiols,38 ro- phologies due to their unique mitaxanes,39 and pseudorotaxanes, active groups were alternated becrophase separations. After which result in surfaces that captween surface-active or -inactive following the selective cross-linking ture and release aromatic guest forms, changing the surface presreaction of core polybutadiene molecules.40 sure, enabling control of the posiblocks, dissolution of the bulk mateAnisotropy-Based Switching. While tion and motion of liquids. In yet anrials yielded anisotropic micelles. all of these examples exploit strucother approach, electrical potential Among the most promising protural transitions in molecular syswas used to trigger conformational cesses currently under evaluation tems with appropriate energy baltransitions of alkanethiols on gold, for fabrication of anisotropic parances between two or more states, while maintaining constant enviticles are methods based on the surfaces that switch based on orien- relatively simple manipulation of ronmental parameters (solvent, tational changes of anisotropic par- liquids followed by rapid solidificaelectrolyte content, pH, temperaticles at an interface can also be en- tion. In this way, multicompartment ture, and pressure).36 A mercaptohexadecanoic acid derivative convisioned. Such an approach requires particles with nonequilibrium matetaining a globular end group was the availability of particles with dis- rials distributions can be fabricated used to form a SAM whose end tinct differences in their appearance in large quantities. Solidification groups were tightly packed, while depending on from which side they must occur sufficiently quickly to maintaining uniform spacing at the are observed (Figure 4). Synthesis prevent mixing between tempogold/thiol interface and in between of anisotropic particles goes well rarily formed compartments. By usthe hydrophobic chains.36 Following flow-focusing technology,79–81 beyond efforts established for
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What sets today’s synthetic materials apart from biological materials is the ability to undergo defined changes in key properties over time.
Figure 4. Representative examples of anisotropic particles with potential for anisotropy-based switching. (A) Painted particles include particles that were selectively modified by gel-trapping,41 metal deposition,42 or layer-by-layer deposition.43 (B) Multicompartment particles include bimetallic nanorods44 and larger microbeads,45 as well as triphasic46 and biphasic nanocolloids. (C) Dumbbell-like particles are made by fusion of different types of particles.47,48 (D) Microstructured particles include particles with controlled topology49 and lithographically modified particles,50 as well as encoded particles51 and those prepared by metal evaporation and chemisorption onto latex particles.62 Caption assignments are made clockwise starting in the upper left quadrant. Image credits are listed clockwise within each respective quadrant. (A) Republished with permission from ref 41, 2004 Wiley-VCH Verlag GmbH & Co.; from ref 42, 2006 Springer; from ref 43, 2005 American Chemical Society. (B) Republished with permission from ref 44, 2003 Macmillan Publishers Ltd.; from ref 45, 2006 Wiley-VCH Verlag GmbH & Co.; from ref 46, 2006 American Chemical Society; from Bhaskar and Lahann, unpublished data. Republished with permission from ref 47, 2004 Wiley-VCH Verlag GmbH & Co.; from ref 48, 2001 American Chemical Society. (D) Republished with permission from ref 49, 2008 American Chemical Society; from ref 50, 2007 The National Academy of Sciences of the United States of America; from ref 51, 2007 American Association for the Advancement of Science; from ref 62, 1997 American Chemical Society. The four center images are original.
microdroplets containing chromophore molecules and biomolecules in separate phases have been generated and cured by appropriate cross-linking mechanisms.82–84 Larger biphasic and multiphasic particles with diameters of 50⫺150 m and narrow size distributions were prepared using microfluidics. Other examples include the fabrication of microparticles using hydrodynamic jets,85–87 or elctrohydrodynamic cojetting.88 A prominent strategy toward anisotropic micro- and nanoparticles uses 1104
electrohydrodynamic cojetting.46,88 In this method, the interface between the two jetting solutions is sustained during jet fragmentation and size reduction. In principle, such novel particle geometries enable independent control of key parameters, such as chemical composition, surface functionalization, biological loading, shape, and size for each compartment, thereby effectively mimicking the architecture of biological cells. This approach toward designer nanoparticles leads to unique capabilities that enable
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the design of particles with multiple and distinct surface patterns or nanocompartments. Because of its intrinsic simplicity and generality, the electrohydrodynamic cojetting process can be applied to a wide range of specialty and nonspecialty materials including many FDA-approved materials.89,90 The fact that each compartment can be designed independently from the other compartments affords anisotropic particles with multiple, orthogonal materials functions. In particular, electrohydrodynamic cojetting can be employed successfully in cases where the functions of incompatible materials must be engineered in close proximity to one another. SUMMARY Major efforts have been undertaken to create increasingly sophisticated materials that start to mimic biological materials with respect to precision, architecture, and functionality. Still, most of today’s materials are intrinsically static. In other words, what sets today’s synthetic materials apart from biological materials is the ability to undergo defined changes in key properties over time. Combinations of theoretical and experimental methods will significantly widen the design parameters available for smart materials. In this regard, the study by Balazs and co-workers is remarkable. Their theoretical results suggest that through simple introduction of www.acsnano.org
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anisotropic particles into lipid bilayers, responsive pores could be created, which can open and close on demand. Although their study focuses on reversible switching due to mechanical forces, alternate triggers such as temperature or pH may also be explored for switching. Beyond the smart membrane studied by Balazs and co-workers, materials containing anisotropic particles can be expected to find applications in fundamental studies of cell architecture as well as in biomedical applications, such as drug delivery or molecular imaging. Despite the impressive advances recently made with switchable materials, even our most basic understanding of surface processes involving molecular switches as well as multicompartment particles is still rudimentary, and much work lies ahead in developing synthetic analogues of switchable systems, such as those used for camouflage by the cuttlefish.
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